The present invention relates generally to protective methods and circuits for sheathed electrical resistance units. More particularly, the invention relates to a method and circuits for reliably preventing or interrupting a fault in the form of an arcing short which may occur between the heating element and the grounded conductive outer sheath of such a heating unit. Conveniently, and by way of example, the invention may be employed in household appliances incorporating heating units such as ranges, hotplates and other cooking appliances, as well as dishwashers.
Sheathed electrical resistance heating units have been employed for many years in various products including, for example, electric ranges and dishwashers. Such heating units generally comprise a heating element in the form of a spiralled, electric resistance wire encased in an elongated, ceramic-filled, metallic outer sheath which is electrically conductive. The ceramic material transmits heat but, in its normal state, is an electrical insulator. Thus, the outer sheath becomes thermally hot, but normally remains electrically insulated from the heating element. A suitable ceramic material is magnesium oxide. Sheathed electrical resistance heating units of this general type are described in U.S. Pat. No. 2,094,480 to Vogel; and U.S. Pat. No. 3,592,771 to Vedder et al, the disclosures of which are hereby incorporated by reference.
In the operation of such heating units, the terminals of the heating element are connected to a power source, for example, a 60 Hz household AC power line which may be 120 volts or 240 volts. A 120 volt heating unit is normally connected between the hot side and neutral of a 120 volt unbalanced to ground line. A 240 volt heating unit is normally connected across both sides of a 240 volt balanced line. Normally, the outer conductive sheath is grounded.
While for the most part such heating units perform satisfactorily, one failure mode which is possible in such a heating unit is associated with a breakdown in the insulation qualities of the magnesium oxide separating the heating element from the outer sheath, accompanied by the formation of a current path between the element and the outer sheath. For reasons not fully presently understood, occasionally a discontinuity develops in the heating element resulting from a physical separation or break in the element itself. This break in the element creates an arc at the discontinuity between the two ends created by the break. The intense heat from the arc rapidly creates a plasma condition in which some of the magnesium oxide material in the vicinity of the arc is vaporized and some is melted. The nature of the magnesium oxide is such that in its solid state it provides a very high resistance; however, in its combined molten liquid and plasma state the resistance becomes relatively less. Thus, as the magnesium oxide melts and vaporizes, a current path of relatively low resistance, is formed from the discontinuity through the molten MgO to the grounded metallic sheath. At this point, the arc transfers from one or both break-created ends of the heating element to the sheath. The force generated by this phenomenon may in some cases actually create a separation or rupture in the outer sheath. Once it starts, such an arc and accompanying disruption in the outer sheath travels lengthwise along the sheath. This phenomenon is commonly referred to as "zippering" because the propagation of the disruption along the outer sheath resembles the opening of a zipper.
One device for rapidly terminating "zippering," should it occur in a heating element of the type employed in a dishwasher, is disclosed in U.S. Pat. No. 4,044,224 issued to Jenkins and Herbst. The Jenkins and Herbst device replaces the direct electrical connection between the outer conductive sheath and ground with a fusible link. During normal operation of the heating unit when the insulation material is intact, substantially no current flows through the outer sheath ground connection (with the exception of a small amount of AC leakage current largely a the result of capacitive effects). However, when a fault occurs, significant current flows through the outer sheath ground connection, causing the fusible link to open. Additional examples of switching and fuse arrangements responsive to changes in current supplied to the heating unit occasioned by ground faults may be found in commonlyassigned U.S. Pat. No. 4,054,857 to Bowling; and commonly-assigned, copending patent application Ser. No. 282,574 to Wellman and Horning, as well as British Pat. No. 946,623 to Ryder. Each of these latter references disclose current actuated switching arrangements employed in ranges and cooking appliances to disconnect power from the heating unit in the event of a ground fault.
Commonly-assigned, copending U.S. patent application Ser. No. 337,887, filed Jan. 7, 1982 by Payne et al describes and claims a protective control arrangement for at least limiting the propagation of the disruption along the length of the sheath. Payne et al periodically deenergizes the heating element regardless of the power setting for the element. Such deenergizations are selected to interrupt any arc which may have developed since this last deenergization and maintain the interruption for a period of time sufficient to allow the molten insulation material to return to a relatively high resistance state in which it once again effectively insulates the heating element from the grounded sheath. The present invention is an improvement over that protective circuit.